In passenger restraint systems of automobiles, for example, proper operation of safety devices such as front, knee, side or head airbags, specifies that a passenger restraint system provided in the vehicle be used by a passenger, and when in use it is also properly locked. Otherwise, the safety apparatus, especially airbag apparatus, in case of collision could even lead to injuries of the affected passenger. Therefore various known devices are used with which the locking state of a tongue of a safety belt inserted into a belt lock can be checked. For example, signals can be generated from the knowledge of the locking state of the belt lock in order to notify the passengers to put on and fasten the safety belt by a signal. Since the introduction of airbag apparatus, information about the locking state of the safety belt systems is also important for activation or deactivation of mechanisms for inflating driver and passenger airbags and knee, side, and head airbags.
To ascertain the locking state of a belt lock known systems propose contactless systems which can be mechanically activated. For example Hall sensors are can be used for contactless monitoring of components which change their position, especially which can assume two different end positions. Hall sensors in principle comprise (e.g., consist of) a semiconductor layer supplied with constant current, in a known integrated construction. The constant current can be influenced by a magnetic field component perpendicular to the semiconductor layer and the sensor delivers a Hall voltage which can be evaluated, which can be tapped and can be used for evaluating a state and also directly as switching voltage. The integrated construction of Hall sensors allows integration of an evaluation circuit which is suitable for evaluating the operating state on the Hall sensor. In the automobile industry therefore Hall sensors are used as contactless state sensors in many applications.
EP-A-0 861 763, for example, discloses a belt lock with an integrated, biased Hall sensor which detects, without contact, the state of a locking body or an ejector for a lock tongue which has been inserted into a belt lock. Here a Hall sensor with a Hall field is located in direct proximity to a permanent magnet. By changing the location of the locking body or of the ejector which for this purpose comprises (e.g., consists of) a ferromagnetic material, the magnetic field of the permanent magnet is changed. Here the signal of the Hall sensor changes and at the output of the Hall sensor the state change can be tapped as a voltage change. In one alternative version it is suggested that the Hall sensor with a Hall field be installed without a permanent magnet and for this purpose the locking body or the ejector be made as permanent magnets. In this arrangement, the change of the position of the locking body or of the ejector should be detectable by a change of the Hall voltage.
The disadvantage in the belt lock design of EP-A-0 861 763 is that the Hall sensor must be positioned very carefully with respect to the locking element or the ejector. Subsequent installation of the Hall sensor is therefore relatively complex and expensive. Depending on its arrangement, the Hall sensor can also be sensitive to stray external magnetic fields which can be caused for example by a magnetic key chain. Optionally even additional shielding must be mounted; this makes mounting or installation even more complicated. The susceptibility to stray external fields is also increased by the signal changes due to the comparatively short distances which are traversed when closing or opening the safety belt lock by the locking body or the ejector being relatively small. The belt lock version without biased Hall sensor, in which either the locking body or the ejector are made as a permanent magnet, is less practicable. The attainable signal changes are also relatively small here; this makes detection of different states difficult, here belt lock open or closed. With time the permanent magnet can be demagnetized due to vibrations of the locking body and of the ejector when the safety belt is open or closed. This can ultimately lead to the Hall sensor becoming ineffective and the state changes of the belt lock no longer being able to be reliably detected.
The known belt locks all have a very compact construction. The space available within the belt lock is therefore generally very limited. This makes it difficult to arrange the sensor components within the belt lock housing, especially in the vicinity of a component which when the belt lock is activated changes its position from one end position into the other end position. Then if shields are also to be mounted, the designer is generally faced with an essentially insoluble problem since the dimensions of the belt lock housing are not to be changed.
In EP-B-1 485 276 a belt lock is described in which the locking state can be checked by a switch which can be mechanically actuated. The switch comprises (e.g., consists of) a fixed contact sheet and a contact sheet which is made as a spring contact and which projects through an opening in one housing wall into the displacement path of a slide which can be moved into two end positions. The slide in the case of locking presses against a middle bent knee region of the spring contact, as a result of which one contact end made in the shape of a hammer comes into contact with the fixed contact sheet. The grinding contact of the slide with the elbow region of the spring contact causes abraded matter which can travel through the opening of the housing wall into the contact region of the contact end and of the fixed contact sheet. This can cause disruptions or in the worst case prevention of the electrical contact in the case of locking. This can even lead to faulty interpretations of the locking state and to misoperations in the following systems, especially the controls of the safety apparatus, such as front, knee, side and/or head airbags.